Anti-factor Xa (anti-Xa) assay.

Area of Science:

• Clinical Chemistry
• Pharmacology
• Hematology

Background:

• The anti-factor Xa (anti-Xa) assay is a functional assay measuring the inhibition of factor Xa by heparin.
• Automated anti-Xa assays have existed since the 1970s but were historically cost-prohibitive for widespread clinical use.
• The assay measures the anticoagulant effect of unfractionated heparin (UFH) and low-molecular-weight heparin (LMWH).

Purpose of the Study:

• To highlight the utility of the anti-Xa assay in monitoring heparin therapy.
• To clarify the distinction between measuring heparin's concentration and its effect.
• To emphasize the role of the anti-Xa assay in guiding therapeutic ranges, including activated partial thromboplastin time (APTT).

Main Methods:

• Functional assay measuring inhibition of factor Xa.
• Utilizes antithrombin (AT) for UFH and direct inhibition for LMWH.
• Automated methods available since the 1970s.

Main Results:

• The anti-Xa assay provides a measure of UFH's anticoagulant effect, not its concentration.
• Cost reductions have made the assay more accessible for clinical management.
• The assay aids in determining therapeutic APTT ranges for UFH.

Conclusions:

• The anti-Xa assay is a valuable tool for monitoring heparin anticoagulant therapy.
• It accurately reflects the therapeutic effect of UFH and LMWH.
• Despite being called a 'heparin assay,' it measures heparin's effect, not concentration.